1. Field of the Invention
The invention relates generally to utility knives with replaceable blades, especially such knives used by those in the construction trades. Such knives include a handle and a blade holder for securing a thin elongated blade, such as a razor blade, in position for cutting. The handle and blade holder are designed to accommodate frequent changing of the razor blade as it dulls during use.
2. Description of the Related Art
Carpet layers have long used replaceable blade knives of various designs for cutting carpet and carpet pad to size during installation of wall-to-wall carpeting. It is important that the cutting blade be razor-sharp to make precise cuts and avoid damaging the carpet. The large number of cuts required and the nature of the materials used in many carpets rapidly dulls the knife blades. Thus, it is necessary to change the razor blades frequently during the course of a job. Other work is similar to carpet installation in that a utility knife is used for cutting materials, and frequent changing of the knife blade is required. Replaceable blade knives, called utility knives, are designed to hold razor blades securely while at the same time permitting easy changing of the razor blades.
Various utility knives are known in the art. U.S. Pat. No. 4,575,940 (Wenzel, 1986), incorporated herein by reference, discloses a utility knife designed for securely holding a razor blade while enabling easy blade changes. Wenzel's knife is designed so that the knife handle may be opened by the loosening of a single thumbscrew. The thumbscrew is driven by a ‘D’ ring in the head of the screw. The ‘D’ ring folds into a recess in the knife handle so that the head of the thumbscrew does not protrude from the handle while the knife is being used. The knife handle includes a compartment for retaining spare blades. The blade in Wenzel's knife is not retractable into the handle.
A knife configured specifically for cutting carpet pad is disclosed by U.S. Pat. No. 4,713,884 (Dunnagan, 1987), which is incorporated herein by reference. Dunnagan's knife is similar to Wenzel's knife. It differs in that the shape of the handle and configuration of the blade are specifically designed for cutting carpet pad instead of carpet or other materials, and in other details. However, it is another example of a non-retractable, replaceable blade knife (like Wenzel's knife) with two handle halves held together by a single connector. Connectors for these types of knives are often designed to be loosened and tightened by hand, eliminating the need for a screwdriver or other tool.
Retractable utility knives are also frequently made with two handle halves that are held together by a single connector. An example is described in U.S. Pat. No. 5,301,428 (Wilcox, 1994). Many similar examples may be found in the art and knives of this type are in widespread commercial use.
An important reason for this two-piece, single connector design for utility knives (of both retractable and non-retractable blade types) is to facilitate replacement of the knife blade. Opening the knife handle unclamps the cutting blade and permits its removal and replacement. Opening the handle also permits access to the spare blades that are frequently stored in the handle of such knives. The conmnonality of the basic two-piece, single connector design across a wide variety of utility knives shows that the design concept is successful in practice, and that ease of blade replacement is an important feature for such knives.
However, blade replacement in prior art utility knives, particularly the opening of the handle halves, is not optimal. Commonly, the two halves of the handle tend to stick together after the connector is loosened. The halves must then be manually separated, and sometimes become suddenly unstuck, causing the razor-sharp blades to spill from the handle. Even when the halves are not stuck together, manual dexterity and mental concentration are required to slide them apart and hold them open the proper distance while the blades are changed. The opening and holding in place preferably is done with one hand to leave the other hand free for changing the blades.
Some utility knives use a compression spring (such as coil, butterfly, or leaf springs) to help lift the handle halves apart. Other knives may use a torsion spring to rotate the halves apart, instead of or in addition to a compression spring. The use of a torsion spring creates two problems in practice.
One problem is maintaining alignment of a torsion spring through many cycles of opening and closing. Incorporating a torsion spring into a knife handle so that it operates reliably and smoothly through many cycles requires precise manufacturing and assembly. This tends to make the knife too costly.
A second problem is that a spring strong enough to overcome the tendency of the handle halves to stick together is too strong to assure a controlled opening of the handle. If the handle becomes unstuck suddenly, the user can lose control of the knife, creating a risk of injury. Also, if the spring is strong enough to reliably open the handle, it will be more difficult for the user to reassemble the handle, because the user must work against the spring. On the other hand, if the spring is too weak, it will not be effective in opening the handle at all.
In general, the use of springs to assist opening of the handle adds complexity to the knife and makes it more difficult to retain the handle in the open position and to reassemble the handle. Springs may also increase the possibility that the user will lose control of the utility knife while opening it.
Thus, there is a need for a reliable means of assisting one-handed separation and opening of utility knife handles that overcomes the disadvantages of springs and does not add too much cost or complexity to the knife.